1. Field of the Invention
This invention relates to the area of electronic components and more specifically to apparatus and method for constructing capacitors with high capacitance and high-energy storage density as well as low equivalent series resistance.
2. Description of Prior Art
Capacitor devices have a host of applications in the electrical, electronics, and microelectronics arts. Many different useful implementations of capacitors were successfully implemented and commercialized. Capacitor properties such as capacitance density, operating voltage, energy storage density, equivalent series resistance (ESR), temperature resilience, and lifetime were constantly improved. Concurrently, substantial drive to reduce the cost and the size of capacitors drove the technology into substantially automatic manufacturing methods and achieved satisfactory commodity status for most applications.
Capacitors are useful for energy storage wherein benefits are fast response, compatibility with high voltage, and extended charge/discharge cycle lifetime (compared to batteries). Most suitable for energy storage and other power applications are electrolytic capacitors that achieve relatively high capacitance density by combining the benefit of a high area anode and corresponding high dielectric constant insulating layer with the contact properties of a liquid or solid electrolyte cathode. The technology of electrolytic capacitors is well known in the art and many useful devices are currently implemented and available in the market. One particular useful design, named Aluminum electrolytic capacitor, applies high area etched aluminum foil with typically between ×25 to ×100 area enhancement factor as an anode and implements the dielectric layer by anodic oxidation growth of Al2O3 layer. The cathode is implemented with additional aluminum foil and the contact between the cathode and the dielectric is typically facilitated by the usage of an electrolytic solution.
Effective incorporation of aluminum electrolytic capacitors into compact devices typically involves winding strips of anode/dielectric foils and a cathode foil separated with a strip of paper or other films suitable for electrolyte impregination into a compact tubular shape followed by impregination with a suitable electrolyte to facilitate the cathode contact.
Aluminum electrolytic capacitors are most commonly used in the industry with advantageous high-capacitance density, relatively high voltage compatibility, and extremely low cost. However, a relatively short lifetime in the order of only several thousands of hours at 85° C., relatively high and constantly deteriorating ESR, high leakage current, polarity, and limited temperature range are only some of the undesired characteristics that have limited the applicability of aluminum electrolytic capacitors, as well as tantalum electrolytic capacitors, as energy storage devices or otherwise circuit components in high performance and highly-reliable electronics. Recent improvements to basic electrolytic capacitor technology successfully incorporate solid polymer electrolytic contact to enhance the lifetime and the useful temperature range with advantageously lower ESR. Clearly, the weak link of aluminum capacitor technology relates to the electrolytic nature of the contact.
Electrolytic capacitors, in general, have been most useful to attain high-capacitance density while they fell short of achieving satisfactory long lifetime, high-voltage compatibility, extended temperature range, and low ESR. In contrast, the technology of thin film capacitors typically implements metalized polymer thin films in an electrostatic capacitor design to achieve significantly suitable high-voltage compatibility, long lifetime, alternating current (AC) compatibility, and improved high-temperature resilience. Thin film capacitors are economically mass-produced by coating both sides of a polymer film with thin metallic films, typically using physical vapor deposition techniques. Compact thin film capacitors are implemented by winding strips of the metalized polymer films into tubular shaped bodies. Alternatively, multilayer stacks of metalized polymer films have been implemented with substantially reduced ESR for the entire capacitor. These film capacitors excel at the high voltage and AC-performance end but have been limited, so far, to relatively low-capacitance density. Additionally, the mainly implemented polymer dielectric films are inherently limited to the temperature range below 120° C. with implications for lower reliability at high-power applications.
The high-capacitance density of electrolytic capacitors is mainly attributed to the starting substrate with its related high-capacitance area. Additionally, a dielectric constant of anodized Al2O3 within an aluminum electrolytic capacitor or an anodized Ta2O5 within Ta electrolytic capacitors at ∈r˜8 and ∈r˜25, respectively, far exceed the typical dielectric constant of ∈r˜2 for suitable polymer films. The capacitance follows the formula:
                    C        =                                            ɛ              r                        ⁢                          ɛ              0                        ⁢            A                    d                                    (        1        )            wherein ∈0 is the permittivity of vacuum, ∈r is the relative dielectric constant of the dielectric material, A is the effective area of the capacitor, and d is the thickness of the dielectric layer. Practically, the thickness of the dielectric layer is determined by the specifications of the voltage that can be reliably applied over the capacitor without causing catastrophic breakdown or deterioration of electrical properties over the lifetime of the capacitor. For example, d=V/EDB where EDB is the dielectric breakdown field of the dielectric layer. In practice, capacitors are typically derated to ensure extended lifetime, and the dielectric thickness is typically extended by a factor of ×1.5-×2.
A schematic layout of aluminum electrolytic capacitor is depicted in FIG. 1. Accordingly capacitor 100 is fabricated by winding a stack of foils 150 into a compact roll with tubular shape. The foils are slit into long strips prior to the winding process. Foils stack 150 includes anode aluminum foil 102 with etched high-area surface 103 and an Al2O3 dielectric layer 104 formed by an anodic oxidation process. Cathode aluminum foil 106 includes a thin layer of Al2O3 108, which is typically substantially thinner than the thickness of dielectric layer 104. The surface of cathode foil 106 is enhanced by etching typically to a much less extent than the area enhancement 103 of anode foil 102. A paper foil 110 is inserted between the anode and the cathode foils prior to winding the capacitors. Foil 110 is soaked with electrolyte solution following the winding, and a cathode contact is formed by electrolyte solution penetrating into gaps 112 and 114 between foil 110 and anode 102 and foil 110 and cathode 106, respectively. Clearly, capacitor ESR relates with the consistency of the electrolyte solution within gaps 112 and 114. The capacitor essentially consists of an equivalent circuit of two capacitors connected in series with the larger capacitor formed on the anode and the smaller capacitor formed on the cathode. These capacitors are mainly suitable for direct current (DC) applications where voltage polarity is substantially maintained positive at the anode.
Electrolytic capacitors typically exhibit continuous deterioration of ESR corresponding to the deterioration of the electrolytic cathode contact. Post fabrication yield improvement relies on the electrolytic solution to further anodize dielectric defects to repair locally cracked and thinned dielectric by the growth of anodic oxide at the localized defect. This growth is enhanced at the defect due to a substantially localized higher current.
Capacitors with a capacitance value typically in the range of 0.01-1 μF are employed in significant numbers on a typical PC board (PCB) to create useful electrical and electronic circuits and, therefore, occupy a significant portion of the PCB area. Additionally, costs related to discrete capacitors assembly over the PCB, as well as yield reduction and failure sometimes related to several hundreds of solder joints, are substantial. Finally, performance limitations related to capacitors to PCB contact resistance and inductance are sometimes difficult to overcome. Accordingly, the electronic industry has pursued the integration of capacitors into capacitor arrays and most recently into the layout of the actual PCBs. Full integration of capacitors into the PCB may advantageously reduce the area that is occupied by the capacitor, further reducing the size of electronic devices. Significant cost and weight reductions are additional benefits. Additionally, performance limitations related to contact resistance and inductance are also foreseen as greatly reduced by this integration.
However, the down sides to integrated capacitors are clearly and obviously the high level of PCB customization that is required and the possible PCB yield reduction relating to defective capacitors. While customization is not foreseen as an issue given the inevitable migration of PCBs into full customization, the industry seeks integration techniques that are compatible with current PCB fabrication technology and that are quickly and easily configurable upon the need to constantly update and advance consumer-electronics products, sometimes within only several months. Integrated capacitors yield, therefore, must be as close as possible to 100% and/or some capacitor redundancy is necessary to support low-cost PCB manufacturing and reduce the insurmountable cost of PCB testing.
There is a need for capacitors with improved energy retention density having both high-capacitance density and high-voltage compatibility while maintaining low ESR. These capacitors should preferably have an extended lifetime at an extended temperature range. Additionally, there is a need to improve the performance and extend the lifetime of high-capacity capacitors and increase the specific capacitance per volume and weight. Also necessary are methods that enable capacitor integration into the layout of PC boards without significantly altering current fabrication techniques while maintaining the ability of existing PC board fabrication lines to quickly and effectively customize their product. In particular, low-cost capacitor device layouts and related fabrication methods are desired.